It has long been known that textiles, such as woven fabrics, knitted fabrics and non-woven fabrics, may be coated with solutions of polyester urethanes by the direct or by the reverse coating processes. The articles obtained are used in the production of outer clothing, upholstery, travel goods, shoe uppers, tarpaulins, blinds, and numerous other products.
In contrast to the two-component polyurethanes which have been known for some time, the so-called "one-component polyurethanes" are fairly recent developments. These products are obtained by reacting polyhydroxyl compounds, especially dihydroxy polyesters or dihydroxy polyethers in admixture with glycols, preferably ethylene glycol or 1,4-butane diol, with aromatic diisocyanates, preferably 4,4'-diphenyl methane diisocyanate. These substantially linear polyurethanes which may be produced both in the melt and in solution are only soluble in solvent mixtures containing dimethyl formamide or other strongly polar compounds, such as dimethyl acetamide or N-methyl pyrrolidone, in quantities of about 20 to 60%, based on the solvent as a whole. The solutions of the one-component polyurethanes have an almost unlimited pot life.
In this case, film formation is merely a physical process which, in contrast to the two-component polyurethanes, is not accompanied by any chemical reaction.
In addition to the use of glycols, such as butane diol, as chain extenders for the production of one-component polyurethanes, the use of diol urethanes, diol amides and diol ureas for producing the hard segments in polyurethanes is also known in the prior art (U.S. Pat. No. 3,388,100).
In addition to the so-called "aromatic" one-component polyurethanes synthesized from aromatic diisocyanates, the so-called "aliphatic" one-component polyurethanes are also known in the prior art. Polyurethanes of this type are polyurethane ureas of relatively high molecular weight dihydroxy compounds, aliphatic isocyanates and aliphatic diamines as chain extenders. In addition, bis-hydrazides, bis-semicarbazides and bis-carbazinic acid esters may also be used as chain extenders.
One-component polyurethanes of dihydroxy polyesters and/or dihydroxy polyethers, aromatic diisocyanates, diols and/or aromatic diamines and/or bis-hydrazides, are used in the form of 20 to 40% solutions in solvent mixtures which always contain large proportions of dimethyl formamide for the production of surface and adhesive coatings by the reverse coating process. One-component polyurethanes based on aliphatic diisocyanates and aliphatic diamines are applied from solvent mixtures which contain secondary or primary alcohols in addition to aromatic hydrocarbons.
Polyurethane elastomers may also be coated onto textile substrates by melt extrusion and may even be processed in the form of aqueous dispersions or in the form of dry sintering powders for textile coating purposes. However, coating with solutions is by far the most widely used prior art process.
According to the prior art, the solvent mixtures in which one-component polyurethanes are dissolved may contain water. According to U.S. Pat. No. 3,432,456, for example, water is used as solvent for the chain extender carbodihydrazide, so that the polyurethane solutions described therein contain from 3 to 4% of water.
German Pat. No. 1,300,273 (corresponding to U.S. Ser. No. 396,998), describes the reaction of NCO-prepolymers in water-containing benzene/acetone mixtures with chain extenders. A water content of from 0.01 to 10% by weight based on the weight of the solution, is proposed in DOS No. 2,229,404 for the purposes of lowering the viscosity of polyurethane solutions. Similarly, the presence of water in polyurethane solutions enables viscosity to be controlled (U.S. Pat. No. 3,428,611). According to DOS No. 1,795,245, water is added in order to obtain polyurethane solutions with reproducible viscosity behavior.
The coating of a textile by the reverse coating process is generally carried out as follows:
The surface-coating solution is coated onto a parting paper in a coating machine, for example, by means of a doctor roll. After the first passage through the drying tunnel, the adhesive-coating solution is similarly applied either in a second coating machine or subsequent return of the web, followed by application of the textile and evaporation of the solvent mixture present in the adhesive coating in the drying tunnel. On leaving the drying tunnel, the parting paper and the coated fabric web are wound into rolls separately from one another.
Considerable difficulties may be encountered in this general sequence of reverse coating onto parting paper, making it impossible to obtain a technically satisfactory coated textile web. When the adhesive-coating solution, which may contain both a one-component polyurethane and also a two-component polyurethane, is applied to the dry, approximately 0.1 mm thick surface coating film, the so-called "frost flower effect" frequently occurs. This descriptive expression is used for the following phenomenon:
If the solvent mixture of the adhesive coating is a poor solvent for the polyurethane of the surface coating, the surface coating swells rather than dissolves. The result of swelling is that, at numerous places over the surface of the web, the surface-coating film is separated from the parting paper, but remains adhering to it at other places. This swelling phenomenon, beginning simultaneously at several places, covers the entire surface in a matter of seconds like freezing frost flowers on a cold window, and makes it unusable for further processing.
According to the prior art, the frost flower effect is counteracted by specifically adjusting the solvent mixture of the adhesive coating (generally dimethyl formamide, methyl ethyl ketone and/or toluene, ethyl acetate, etc.) in its dissolving power to the solubility of the polyurethane used for surface coating. In practice, this generally involves increasing the dimethyl formamide content of the adhesive-coating solution in order to increase its dissolving power. If the solvent mixture of the adhesive coating contains enough effective solvent, for example, 30 to 60% of dimethyl formamide, the surface coating is not swollen, but lightly dissolved, thereby suppressing the frost flower effect.
If, on the other hand, the dissolving power of the adhesive-coating solution is too great for the surface coating, the so-called "break-through" effect is observed. The already dry surface coating is dissolved by the solvent mixture used for the adhesive coating to such an extent that the web of textile material applied is forced through both layers of polyurethane, with the result that the textile structure, for example pile, is visible on the top of the coating. An adhesive-coating solution always has too high a dissolving power for the surface coating when it contains too much, or nothing but, dimethyl formamide or other polar solvents.
The protection of the environment by the reduction of atmospheric pollution by the combustion or recovery of organic solvents in industrial processes, such as the coating of textiles with polyurethane solutions, is an acute industrial problem.
The ecologically necessary recovery of solvent from the textile coating process may only be carried out rationally and economically if, contrary to the prior art, the polyurethane solutions contain a single solvent as opposed to solvent mixtures, such as dimethyl formamide/methyl ethyl ketone, dimethyl formamide/methyl ethyl ketone/toluene, toluene/isopropanol, etc.